merge-tree: accept 3 trees as arguments

When specifying a merge base explicitly, there is actually no good
reason why the inputs need to be commits: that's only needed if the
merge base has to be deduced from the commit graph.

This commit is best viewed with `--color-moved
--color-moved-ws=allow-indentation-change`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-tree.txt

@@ -64,10 +64,13 @@ OPTIONS
 	share no common history.  This flag can be given to override that
 	check and make the merge proceed anyway.
 
---merge-base=<commit>::
+--merge-base=<tree-ish>::
 	Instead of finding the merge-bases for <branch1> and <branch2>,
 	specify a merge-base for the merge, and specifying multiple bases is
 	currently not supported. This option is incompatible with `--stdin`.
++
+As the merge-base is provided directly, <branch1> and <branch2> do not need
+to specify commits; trees are enough.
 
 [[OUTPUT]]
 OUTPUT

builtin/merge-tree.c

@@ -430,35 +430,43 @@ static int real_merge(struct merge_tree_options *o,
 	struct merge_options opt;
 
 	copy_merge_options(&opt, &o->merge_options);
-	parent1 = get_merge_parent(branch1);
-	if (!parent1)
-		help_unknown_ref(branch1, "merge-tree",
-				 _("not something we can merge"));
-
-	parent2 = get_merge_parent(branch2);
-	if (!parent2)
-		help_unknown_ref(branch2, "merge-tree",
-				 _("not something we can merge"));
-
 	opt.show_rename_progress = 0;
 
 	opt.branch1 = branch1;
 	opt.branch2 = branch2;
 
 	if (merge_base) {
-		struct commit *base_commit;
 		struct tree *base_tree, *parent1_tree, *parent2_tree;
 
-		base_commit = lookup_commit_reference_by_name(merge_base);
+		/*
-		if (!base_commit)
+		 * We actually only need the trees because we already
-			die(_("could not lookup commit '%s'"), merge_base);
+		 * have a merge base.
+		 */
+		struct object_id base_oid, head_oid, merge_oid;
+
+		if (repo_get_oid_treeish(the_repository, merge_base, &base_oid))
+			die(_("could not parse as tree '%s'"), merge_base);
+		base_tree = parse_tree_indirect(&base_oid);
+		if (repo_get_oid_treeish(the_repository, branch1, &head_oid))
+			die(_("could not parse as tree '%s'"), branch1);
+		parent1_tree = parse_tree_indirect(&head_oid);
+		if (repo_get_oid_treeish(the_repository, branch2, &merge_oid))
+			die(_("could not parse as tree '%s'"), branch2);
+		parent2_tree = parse_tree_indirect(&merge_oid);
 
 		opt.ancestor = merge_base;
-		base_tree = repo_get_commit_tree(the_repository, base_commit);
-		parent1_tree = repo_get_commit_tree(the_repository, parent1);
-		parent2_tree = repo_get_commit_tree(the_repository, parent2);
 		merge_incore_nonrecursive(&opt, base_tree, parent1_tree, parent2_tree, &result);
 	} else {
+		parent1 = get_merge_parent(branch1);
+		if (!parent1)
+			help_unknown_ref(branch1, "merge-tree",
+					 _("not something we can merge"));
+
+		parent2 = get_merge_parent(branch2);
+		if (!parent2)
+			help_unknown_ref(branch2, "merge-tree",
+					 _("not something we can merge"));
+
 		/*
 		 * Get the merge bases, in reverse order; see comment above
 		 * merge_incore_recursive in merge-ort.h

t/t4301-merge-tree-write-tree.sh

@@ -945,4 +945,10 @@ test_expect_success 'check the input format when --stdin is passed' '
 	test_cmp expect actual
 '
 
+test_expect_success '--merge-base with tree OIDs' '
+	git merge-tree --merge-base=side1^ side1 side3 >with-commits &&
+	git merge-tree --merge-base=side1^^{tree} side1^{tree} side3^{tree} >with-trees &&
+	test_cmp with-commits with-trees
+'
+
 test_done